Active Memory Expansion - IBM BladeCenter PS703 Technical Overview And Introduction

2.5 Active Memory Expansion

Optional Active Memory Expansion is a POWER7 technology that allows the effective
maximum memory capacity to be much larger than the true physical memory. Innovative
compression/decompression of memory content using processor cycles can allow memory
expansion up to 100%.
This can allow an AIX 6.1 or later partition to do significantly more work with the same
physical amount of memory, or a server to run more partitions and do more work with the
same physical amount of memory.
Active Memory Expansion uses CPU resources to compress/decompress the memory
contents. The trade off of memory capacity for processor cycles can be an excellent choice,
but the degree of expansion varies based on how compressible the memory content is, and it
also depends on having adequate spare CPU capacity available for this compression/
decompression. Tests in IBM laboratories using sample workloads showed excellent results
for many workloads in terms of memory expansion per additional CPU utilized. Other test
workloads had more modest results.
Clients have a great deal of control over Active Memory Expansion usage. Each individual
AIX partition can turn on or turn off Active Memory Expansion. Control parameters set the
amount of expansion desired in each partition to help control the amount of CPU used by the
Active Memory Expansion function. An IPL is required for the specific partition that is turning
memory expansion on or off. After being turned on, there are monitoring capabilities in
standard AIX performance tools such as lparstat, vmstat, topas, and svmon.
Figure 2-10 represents the percentage of CPU used to compress memory for two partitions
with various profiles. The green curve corresponds to a partition that has spare processing
power capacity, while the blue curve corresponds to a partition constrained in processing
power.
% CPU
utilization
for
expansion
Figure 2-10 CPU usage versus memory expansion effectiveness
Both cases shows that there is a knee-of-curve relationship for CPU resources required for
memory expansion:
Busy processor cores do not have resources to spare for expansion.
The more memory expansion is done, the more CPU resource is required.
The knee varies depending on how compressible memory contents are. This demonstrates
the need for a case by case study of whether memory expansion can provide a positive return
on investment.
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IBM BladeCenter PS703 and PS704 Technical Overview and Introduction
Very cost effective
Amount of memory expansion
2
1
1 = Plenty of spare
CPU resource available
2 = Constrained CPU
resource – already
running at significant
utilization